1. Field of the Invention
This is a method for estimating the principal axis of aximuthal anisotropy by use of compressional seismic wavefields. For many crusted materials, the azimuthal anisotropy is induced by vertical fractures, in which case the principal axes corresponds to the orientation of fractures.
2. Discussion of Related Art
Fracture detection is of great interest in seismic exploration. Because of improved technology in horizontal drilling, determination of vertical fracture orientation is important. If fracture orientation can be measured from seismic data, horizontal boreholes can be directed perpendicular to the fractures for maximum hydrocarbon recovery. Vertical fractures are formed in relatively brittle subsurface earth formations due to folding stresses. A typical example is the fractured Austin Chalk which underlies the Taylor shale formation.
It is well known that vertical fractures in an otherwise isotropic rock will induce angular or azimuthal anisotropy in the fractured medium. By definition, in an isotropic medium, the velocity of propagation of an acoustic wave is the same along all three spatial axes: x, y, and z. In an anisotropic medium, the acoustic velocity is greater along one axis, termed the principal axis, than it is along the other two axes. The velocity increase may be due to a change in the elastic constants of the rock such as that due to a change in cementation, particle size, composition, depth of burial, fluid content, structure and other factors. Commonly, the principal anisotropic axis is along the vertical or z axis. Lateral anisotropy along a substantially horizontal axis sometimes is observed on a broad regional basis but that change is gradual and of little concern on a local basis.
As stated earlier, vertical, oriented fractures in an otherwise isotropic medium induce significant local lateral anisotropy fields. The propagation velocity of a wavefield varies with the ray-path incident angle and the lateral angular direction of wavefield propagation relative to the strike of the fracture zone. The velocity of an acoustic wavefield is greatest along a direction parallel to strike and is least when the wavefield trajectory is perpendicular to the plane of the vertical fracture zone.
An explanation of the physical reasons for that effect may be found in U.S. Pat. No. 4,817,061 which issued Mar. 28, 1989 to R. M. Alford et al. which is incorporated herein by reference, but only for its tutorial content. In that patent, polarized shear wave surveys are performed to determine azimuthal variations in the earth's subsurface caused by fracture orientation and density. The surveys may be done by using the same polarization (either horizontal or vertical) for the shear waves along two different seismic lines of profile or by using two different polarizations along a single common line of profile. The survey data can then be processed and compared; any difference constitutes a measure of fracture orientation and density. That patent, however, teaches away from and specifically dismisses use of compressional or P waves for vertical fracture studies.
The method taught by the '061 patent has the serious disadvantage that multicomponent sources and receivers must be used in order to generate shear waves having the required polarization. That drawback is particularly disadvantageous in the case of the sources because two separate vibrators must be used, one to generate horizontally-polarized shear waves and one to generate vertically-polarized wavefields, each vibrator costing several megadollars.
U.S. Pat. No. 4,571,710, issued Feb. 8, 1986 to N. S. Neidel et al for a Seismic Method for Identifying Low Velocity Subsurface Zones, teaches a method which uses seismic data having different directions of propagation through a common subsurface anomaly to measure the interval velocity variation within a selected formation interval, using moveout-derived velocities as a function of direction. A velocity variation is taken to be qualitatively diagnostic of porosity.
The '710 patent is of limited use for qualitative measures of porosity but the method is not uniquely indicative of vertical fracturing.
In an effort to reduce the high cost of the sources needed for the '061 patent method, Mallick and Frazer suggested use of converted waves from P-wave data in a paper entitled Reflection/Transmission Coefficients and Azimuthal Anisotropy in Marine Seismic Studies, published in the Geophysical Journal International, v. 105, pp 241-252, 1991. Use of converted wave data does indeed provide a cheaper alternative to a multicomponent wavefield source. But the signal-to-noise ratio in converted waves data is usually very low which causes serious errors in predicting the fracture orientation. Moreover, the method requires dense 3-D multicomponent recording resulting in an extremely large volume of data that needs to be processed as well as new interpretation techniques that have not yet been fully developed by the seismic industry.